1. Field of the Invention
The present invention relates to a multi-level information reproducing method of reproducing multi-level information recorded on an information recording medium.
2. Related Background Art
In recent years, the optical memory industry is growing, and read-only recording mediums including CDs and DVDs have been developed, and in addition, not only a write-once, read-many recording medium using a metal thin film or a dye recording material but also a rewritable recording medium using a magneto-optical material or a phase change material have been developed, and the applications are expanding from consumer devices to external memories of computers. Further, research and development for increasing the density of a recording capacity are under way, and a technique for realizing a very small size of a light spot for information recording/reproducing is being developed to improve the wavelength of a light source from a red region (650 nm) to a blue-violet region (405 nm). Moreover, the numerical aperture of an objective lens is also being increased from 0.6 or 0.65 to 0.85. On the other hand, a technique for more efficiently performing multi-level recording/reproducing using a light spot of the same size has been proposed.
For example, the present inventors have already proposed a technique related to the multi-level recording/reproducing in Japanese Patent Application Laid-open No. H05-128530. That is, multi-level information is recorded on an information track of an optical information recording medium based on a combination of a width of an information pit in a track direction and a shift amount of the information pit in the track direction relative to a reproducing light spot. Then, the recorded multi-level information is reproduced from the information pit based on a correlation between a detection signal learned in advance and a detection signal obtained from the light spot.
In the meeting of ISOM 2003, which is an international society of research in an optical disk field, a report has been presented on having performed multi-level recording/reproducing of an eight-level (“Write-once Disks for Multi-Level Optical Recording”, Extended abstract Fr-Po-04). To be specific, a blue-violet light source (405 nm) and an optical system whose NA is 0.65 are used. In the case of an optical disk whose track pitch is 0.46 μm, the width of a region which is virtually provided to include an information pit for recording (hereinafter referred to as a cell) in the track direction is set to 0.26 μm.
For example, as shown in FIG. 14, selection of eight-level information pits is performed by divide the width of a cell in a track direction (direction indicated by arrow A in FIG. 14) into 16 equal parts. Then, it is defined that level 0 indicates that no information pit is recorded; level 1 indicates 2/16 of the width of the cell; A level 2 indicates 4/16 of the width of the cell. A level 3 indicates 6/16 of the width of the cell. A level 4 indicates 8/16 of the width of the cell. A level 5 indicates 10/16 of the width of the cell. A level 6 indicates 12/16 of the width of the cell. A level 7 indicates 14/16 of the width of the cell.
The thus selected information pits are recorded at random. Further, a reproduced signal from the information pit is sampled at a timing when the center of a light spot is positioned at the center of the width of the cell in the track direction. As a result, the amplitudes of reproduced signals corresponding to the respective levels are obtained in a distribution as shown in FIG. 15.
Here, normalization is performed when the level 0 in which no information pit for writing is formed is repeated, an amplitude of the reproduced signal is as “1”. In addition, when the information pit for recording corresponding to the level 7 is repeated, an amplitude of the reproduced signal is normalized as “0”.
The values of reproduced signals corresponding to respective levels have a width because of the influence of information pits written before and after an information pit of question (inter-symbol interference). When the amplitude distribution of reproduced signals of a level overlaps that of an adjacent level as shown in FIG. 15, the levels cannot be separately detected based on a fixed threshold value.
In order to solve this problem, the following separation/detection system is described in the report presented in the ISOM2003 meeting. Reproduced signals of a pit string in which a value of an information pit of question and values of information pits which precede and follow the information pit of question are known in advance are read and then stored (learning). Then, reproduced signals from actual information pits are compared with the stored values (correlation) to thereby perform separate detection. In this system, the recording density is approximately 16 Gbit/inch2.
Further, a higher recording density of approximately 30 Gbit/inch2 is to be attained based on the multi-level system presented in the ISOM2003 meeting, by using a blue-violet light source (405 nm) and an optical system with an NA of 0.85 thereby making a light spot finer, there is posed the problem that the amount of inter-symbol interference becomes larger, whereby the overlapping between amplitude distributions of reproduced signals for respective levels shown in FIG. 15 becomes more prominent to thereby increase reproducing errors.